In recent years, a technique of decomposing water by using a photocatalyst and solar energy so as to producing hydrogen and oxygen has attracted attention. The photocatalyst which has been currently studied is ordinarily obtained by loading a co-catalyst on a surface of an optical semiconductor such as an oxide, an oxynitride, or a nitride. It is possible to improve the activity of the photocatalyst by loading the co-catalyst (for example, refer to NPLs 1 and 2).
As a method of loading the co-catalyst on the surface of the optical semiconductor, an impregnation loading method has been known (for example, refer to PTL 1). Various methods in addition to the impregnation loading method have been proposed (for example, refer to NPLs 3 and 4).
On the other hand, when the photocatalyst is formed by using the optical semiconductor, the surface of the optical semiconductor is subjected to an acid treatment by using an inorganic acid in advance (refer to PTLs 2 to 4, NPL 5, and the like). For example, in PTL 2, an H-type layered perovskite photocatalyst is obtained through the acid treatment performed on an auribilian phase compound by using an inorganic acid such as a hydrochloric acid. In addition, in PTL 3, the photocatalytic activity is improved through the acid treatment performed on the photocatalyst represented by General formula (I): ABCO4 (A represents silver, B represents at least one or more types of elements selected from the group consisting of the lanthanoid and yttrium, and C represents at least one or more types of elements selected from a Group IVa element) by using a nitric acid having a low concentration. In addition, in PTL 4, the photocatalytic activity is improved through a mineral acid treatment (inorganic acid treatment) performed on a titanium oxide. Further, in NPL 5, the water splitting activity in a case where the photocatalyst is obtained by bringing aqua regia into contact with the surface of the optical semiconductor for 15 seconds is improved.